http2: refactor multiple internals

* eliminate pooling of Nghttp2Stream instances. After testing,
  the pooling is not having any tangible benefit
  and makes things more complicated. Simplify. Simplify.

* refactor inbound headers

* Enforce MAX_HEADERS_LIST setting and limit the number of header
  pairs accepted from the peer. Use the ENHANCE_YOUR_CALM error
  code when receiving either too many headers or too many octets.
  Use a vector to store the headers instead of a queue

PR-URL: https://github.com/nodejs/node/pull/16676


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>